title: Zygosity and chorionicity in triplet pregnancies: new data.
abstract: BACKGROUND
  In studies of twin and triplet pregnancies, molecular genetic techniques have rarely been used to confirm zygosity yet this is the most accurate approach. The aim of this study was to present the cross-distribution of chorionicity and zygosity in triplet pregnancies as a function of their mode of conception using such techniques. Rates of monozygosity were studied simultaneously.
  
  
  METHODS
  Forty-nine consecutive sets of triplets were observed in the study, including 18 sets of spontaneously conceived (SC) triplet pregnancies and 31 sets resulting from assisted reproduction technologies (ARTs). Zygosity was determined through PCR-amplified microsatellite analysis. Chorionicity was determined by placental analysis in our department of fetopathology. Sets of triplets were considered as twin pairs in order to determine the rate of monozygosity.
  
  
  RESULTS
  For SC triplet pregnancies, the rate of monozygotic (MZ) twin pairs was 48%; 30% of dichorionic (DC) triplet pregnancies were MZ and 70% dizygotic (DZ); 20% of trichorionic (TC) triplet pregnancies were DZ and 80% trizygotic (TZ). For triplet pregnancies conceived using ART, the rate of MZ twin pairs was 6.5%; 100% of DC triplet pregnancies were DZ; 4% of TC triplet pregnancies were DZ and 96% TZ.
  
  
  CONCLUSIONS
  This study is the first report to present the cross-distribution of chorionicity and zygosity in triplet pregnancies as a function of their mode of conception. In triplet pregnancies conceived using ART, DC triplets are always DZ, and TC triplets are almost always TZ.
authors.affiliation: Department of Prenatal Diagnosis, Robert Debre Hospital, Paris, France. romain.guilherme@psl.aphp.fr
authors.forename: Romain
authors.initials: R
authors.lastname: Guilherme
authors.forename: Séverine
authors.initials: S
authors.lastname: Drunat
authors.forename: Anne-lise
authors.initials: AL
authors.lastname: Delezoide
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Oury
authors.forename: Dominique
authors.initials: D
authors.lastname: Luton
mesh_terms: D002823:Chorion; D005260:Female; D005306:Fertilization; D006801:Humans; D018895:Microsatellite Repeats; D011110:Polymorphism, Genetic; D011247:Pregnancy; D011272:Pregnancy, Multiple / Q000235:genetics*; D027724:Reproductive Techniques, Assisted; D014310:Triplets / Q000235:genetics; D015053:Zygote